1. Field of the Invention
The present invention concerns a method for localization of an epileptic focus in neuroimaging.
2. Description of the Prior Art
The following definitions, acronyms, and abbreviations are used herein:    ECD Ethyldysteinatdimers    HMPAO Hexamethylpropyleneamine Oxime    MRI Magnetic Resonance Imaging    PET Positron Emission Tomography    SISCOM Subtraction Ictal SPECT Co-registered to MRI    SPECT Single-Photon Emission Computed Tomography
Epilepsy is a brain disorder in which a person has repeated seizures over time. Approximately 1% of the UK population is affected by epilepsy which is usually controlled (but not cured) with medication. However, 25-30% of people with epilepsy do not have seizure control even with the best available medications.
Surgery may be considered in difficult cases, where a required evaluation will include neurological examination, routine EEG, Long-term video-EEG monitoring, neuropsychological evaluation, and neuroimaging using modalities such as MRI, SPECT and PET.
Once a patient is considered for surgery, the exact location of the epileptic focus needs to be established. Functional SPECT imaging is one imaging technique that could reveal alterations in cerebral metabolism. Images captured during a seizure are referred to as “ictal” or “intervention”, while those captured between seizures are referred to as “interictal” or “baseline”. Ictal SPECT studies can be obtained by injecting appropriate radioisotope (99mTc-ECD or 99mTc-HMPAO) within seconds of a seizure onset to measure cerebral perfusion. Epileptic areas are hypometabolic between seizures and hypermetabolic during seizures.
One way to facilitate the localization of an epileptic focus is to subtract an interictal image from a corresponding ictal image after alignment and intensity normalization. The resulting difference image identifies areas showing significantly different ictal and interictal uptakes. The identified area may be overlaid on a corresponding MRI image to find the anatomical correspondence. This is called the Subtraction Ictal SPECT Co-registered to MRI (SISCOM) Zubal et al., “Difference images calculated from ictal and interictal technetium-99m-HMPAO SPECT scans of epilepsy,” Journal of Nuclear Medicine, Vol. 36: (1995) pp. 684-689.
Accordingly, one way to facilitate the localization of an epileptic focus in SPECT imaging is to analyze a corresponding SPECT difference image. Once the intervention and baseline images (ictal and interictal images) are registered and intensity normalized, the image data are subtracted, preferably only in the area of the brain outline.
The difference image acquired in this way is beneficial in helping clinicians in identifying epileptic foci O'Brien et al. “Subtraction ictal SPECT co-registered to MRI improves clinical usefulness of SPECT in localizing the surgical seizure focus,” Neurology, Vol. 50:(1998) pp 445-454. The difference image will have regions with both positive and negative variations. These variations could help in identifying areas showing significantly different uptake. However, difference images acquired according to current methods can be difficult to interpret as they may show too many potential areas of high variation, most of which are in fact irrelevant, and distracting.
Few techniques exist to facilitate the location of epileptic foci. The oldest proposes the classical SISCOM method mentioned above [Zubal1995, O'Brien1998]. To help in the analysis, the difference image is first normalized to a Z-score image, that is to say, a mean value of the image data in the region of interest is calculated. In the present context, the region of interest is the image of the brain. The calculated mean value is then subtracted across the difference image and the resulting value of each pixel divided by the standard deviation of the difference in the brain only. Then, a threshold, corresponding to a number of standard deviations, is selected to visualize and delineate potential foci of interest. However, this has often been found to identify too many potential focus areas. In the references Zubal1995 and O'Brien1998 no further processing is proposed on the difference image to facilitate its interpretation.
More recently-developed methods use ictal SPECT analysis by SPM (statistical parametric mapping), that is, by comparing ictal-interictal pairs to normal images stored in a reference database Lee et al., “Evaluation of ictal brain SPET using statistical parametric mapping in temporal lobe epilepsy,”. European Journal of Nuclear Medicine, Vol. 27, (2000) pp 1658-1665 or use ictal-interictal SPECT analyzed by SPM (statistical parametric mapping), that is, by comparing ictal-interictal pairs to pairs of normal images stored in a reference database. Such techniques are described in McNally et al. “Localizing value of ictal-interictal SPECT analyzed by SPM (ISAS),” Epilepsia, 46(9):1450-64 (2005) and Chang et al. “Comparison of Statistical Parametric Mapping and SPECT Difference Imaging in Patients with Temporal Lobe Epilepsy,” Epilepsia, 43:68-74 (2002).